The present invention relates to the printing industry and more specifically to the verification of data files before generating a printing plate. Such verification, known as proofing, is normally done by printing out the file using a laser printer, color printer or similar low resolution device. When films are used to make plates the films can be used for proofing before plates are imaged and printed. With the trend to eliminate use of film, a trend known as "Computer-to-Plate" systems, film is no longer available for proofing. All solutions generating a low resolution paper proofs, both color and monochrome, have common drawbacks. First, the cost of materials is significant for color proofs. Second, the time it takes to print out a color proof is significant. Third, due to cost limitations, the resolution of these proofing devices is too low in order to accurately measure registration errors and color overlap (trapping") errors.
Software based proofing is available (reading a document on a computer before printing it out is proofing) but prior art does not fill the needs of the printer. The shortcomings of prior art software proofing are: First, most systems proof from the input, typically PostScript, data (PostScript is a trademark of Adobe and is the most common input format in the printing business). An accurate proof requires looking at the rasterized data instead of PostScript as errors can occur in the rasterizing for the output device (rasterizing takes place, of course, in the display but it uses a different rasterizing engine and algorithm than the output device). Second, PostScript does not allow the front-to-back overlay of two files printed on both sides of a sheet of paper. This front-to-back registration is one of the crucial items to be proofed. Third, most image viewing programs operate on the composite color files which does not allow the ability to selectively control which colors are displayed together. It is an object of the present invention to overcome the limitations of prior art software based proofing and bring it to performance level equivalent to paper proofs without the delays and costs associated with the latter. A further object is to accurately measure the registration errors and produce correction data in order to eliminate the need for many proofs. These and other objects of the present invention will become apparent from reading the following description in conjunction with the drawing.